Generally, bicycle speed change hubs are so constructed that a hub shell and a driving member having a sprocket wheel for engaging a drive chain are supported rotatably on a hub shaft fixed to the bicycle frame. The hub shell houses a speed change gear mechanism provided with a sun gear mounted to the hub shell; a ring gear rotatable in association with the driving member and having first transmitting members engageable with the hub shell; a gear frame having planetary gears in mesh with inner teeth of the ring gear and the sun gear and having second transmitting members engageable with the hub shell; and a relay member rotatable in association with the driving member, having third transmitting members engageable with the gear frame, supported axially slidably to the hub shaft, and having clutch means to control the first transmitting members to engage with or disengage from the hub shell. The hub shell slidably supports an operating rod which is operated to axially slide the relay member, so that the first or third transmitting members are controlled to engage with or disengage from the hub shell, thereby transmitting the driving force thereto at high or low speed or directly.
For the high speed mode, the first transmitting members engage with inner teeth at the hub shell and the third transmitting members engage with inner teeth at the gear frame, thereby transmitting the driving force from the driving member to the hub shell through the relay member, third transmitting members, gear frame, planetary gears, ring gear and first transmitting members. In a case where the bicycle is switched from the high speed mode to the middle speed mode, the relay member is actuated to release the first transmitting members from the hub shell, thereby transmitting the driving force from the driving member to the hub shell through the relay member, third transmitting members, gear frame and second transmitting members. In a case of switching from the middle speed mode to the low speed mode, the relay member is further actuated to disengage the third transmitting members from the ring gear while keeping the first transmitting members disengaged from the hub shell, thereby transmitting the driving force to the hub shell through the ring gear, planetary gears, gear frame and second transmitting members.
However, when the bicycle is switched from the high speed mode to the low speed mode, the relay member is subjected to a load transmitted from the rear wheel to the hub shell through the third transmitting members, whereby a sliding resistance of the relay member is very great. As a result, an extremely intense operating force is required, thereby resulting in diminished speed change efficiency.